1. Field of the Invention
The present invention relates to a small-sized, light-weight DC torque motor which has good response characteristics and is simple to control, a drive control apparatus using the DC torque motor and a throttle valve control apparatus for an internal combustion engine.
2. Description of the Related Art
Conventionally, there has known a torque motor being rocked within a predetermined working angle range. For example, as shown in FIG. 11, Japanese Unexamined Patent Publication 6-6964 discloses a DC torque motor 100 consisting of a needle 101 rockable around axis 103 and a generally U-shaped stator 102 with a cavity disposed between them, wherein the needle 101 is composed of a soft iron core 104 and two permanent magnets 105, 106 fixedly attached to the periphery of the core 104, and the stator 102 has polarity portions 109, 110 facing to each other and coil 112 is wound around the proximal portion of the stator 102 (the upper part in FIG. 11).
In the above-specified DC torque motor and the like, torque is designed to be almost constant irrespectively of degree of angle within a predetermined angle range on condition that the current flowing in the coil is set constant. That is, the torque is designed to be flat to any angles within the predetermined angle range. For example, the above-mentioned publication discloses a DC torque motor capable of securing constant torque with constant flowing of current at almost entire ratable positions for the needle.
However, section area of the stator 102 (section area of magnetic path) needs to be large so as to achieve such characteristics. The stator 102 is affected by magnetic fields of the permanent magnets 105, 106 fixedly attached to the needle 101 and a magnetic field generated by current flowing in the coil 112. However, directions of the above-stated magnetic fields may sometimes coincide with each other, which depends on angle a of the needle 101. Under such a condition, since the stator 102 is made of soft magnetic material such as soft iron, once fluxoid quantum is saturated, the fluxoid quantum does not increase greatly even when current flows in the coil. As a result, torque becomes low in proportion to a variation of the fluxoid quantum. Therefore, section area of magnetic path needs to be made large to avoid saturation of the fluxoid quantum.
However, as the section area of magnetic path for the stator 102 is enlarged, dimensions, volume, weight of the stator 102 increase proportionally. Further, in case that a DC torque motor is controlled by angle of its shaft (rocking angle) .alpha., large torque is required at the initial rocking so that the DC torque motor can be rocked within a large angle range to obtain high response characteristics. However, large torque is not required at the near-to-end rocking. Further, considerably high torque is not required for the DC torque motor to be rocked by a small angle.